1. Field of the Invention
The present invention relates to a device that detects failure in a power semiconductor element employed in an inverter circuit and the like.
2. Description of the Background Art
There is known the art of sensing overcurrent at a power semiconductor element to obviate damage of the semiconductor element, as disclosed in Japanese Patent Laying-Open Nos. 04-185228 and 2001-197724.
The short-circuit protection device disclosed in Japanese Patent Laying-Open No. 04-185228 is configured having, parallel to a switching element constituting an inverter, a low voltage supply for short-circuit detection, a variable resistor, and a diode applying forward bias to the switching element by the low voltage supply only when the switching element is conductive, connected in series. As a result of a flow of high current to the switching element, the voltage output from the voltage output terminal of the variable resistor will exceed the threshold value. At this stage, the switching element is forced to be cut off, whereby damage at the switching element is obviated.
At a gate drive circuit of an IGBT (Insulated Gate Bipolar Transistor) disclosed in Japanese Patent Laying-Open No. 2001-197724, the voltage across the collector and emitter of the IGBT is detected via the resistor and diode. Determination is made that the IGBT attains an overcurrent state when the detected value exceeds the voltage of the reference power supply incorporated in the overcurrent determination circuit. In response, the cutoff circuit is operated to gradually turn OFF the IGBT.
In the case where the switching element for power such as the IGBT or MOSFET (Metal Oxide Semiconductor Field Effect Transistor) constituting the power module fails by some reason, the possibility of short-circuiting between the main electrodes of the switching element is high. In the event that the load coupled to the power module corresponds to inductive load such as a synchronous motor, the counter-electromotive force generated at the load by the short-circuit current across the main electrodes may cause overload at the power module.
In order to protect the entire system employing a power module, the breakdown voltage of the power switching element must be constantly monitored to cut off the current output from the power module to the load when the switching element breaks down.